After backing up or when parked on a decline, you may find it impossible to un-hook the trailer from the two vehicle. This is due to the fact that the brake mechanism is in a pushed in or activated condition which will not allow un-hitching.
First, block the trailer tires front and back. Next roller the two vehicle forward slightly to allow the brake actuator to completely unload pressure against the brake unit on the trailer. Now you should be able to lift the release and disconnect the trailer.